(a) Field of the Invention
The present invention relates to a reticle system for use in measurement of effective coherence factors ("sgr") in the exposure field of an exposure apparatus to calculate the dispersion of the effective coherent factors, and to a method for calculating the dispersion of the effective coherent factors by using the reticle system.
More specifically, the present invention relates to a reticle system that can accurately measure the effective coherent factors in the exposure field of an exposure apparatus without difficulty to calculate the dispersion of the effective coherent factors, and to a method for calculating the dispersion of the effective coherent factors.
(b) Description of the Related Art
Manufacture of semiconductor devices typically involves formation of a photoresist film on a wafer. An etching mask having thereon a circuit pattern is formed using a photolithographic step for the photoresist film, followed by etching the underlying layer by using the etching mask to form a circuit pattern on the wafer surface.
Typically, in the photolithographic process, an exposure apparatus is used to transfer a circuit pattern onto the photoresist film. Referring to FIG. 1, the exposure apparatus 100 includes a light source 101 such as a halogen lamp, a mirror system 102 for passing the light emitted from the light source 101, a condensing lens system 103 for focusing the light passed by the mirror system 102, a reticle 104 disposed on a reticle holder 105 in the exposure field of the exposure system 100, and a reducing lens system 106 for projecting the light that has passed through the reticle 104 onto a wafer 107 mounted on a stage 108.
Aberrations in the optical systems of the exposure apparatus 100, such as the condensing lens system 103 and the reducing lens system 106, cause influences on the process margin and dimensional dispersion in the exposure field of the reticle 104.
As semiconductor devices become more integrated and have smaller dimensions, the design rule for the semiconductor devices is on the verge of the resolution limit of the exposure apparatus. Thus, the influence by the aberrations in the optical system of the exposure apparatus becomes an increasingly serious problem.
In order to efficiently reduce the optical aberration, it is important to develop a method for accurately measuring the dispersion of the effective coherence factors in the exposure field.
The coherence factor represents the effective size or scale of the optical system. The dispersion of the coherence factors in the exposure field means the variations in the coherence factor at different positions inside the exposure field for each exposure shot. In consideration that coherence factor is a parameter that determines the image contrast, the dispersion of the coherence factors in the exposure field means the variations of exposure performance attained in the exposure field.
The dispersion of coherence factors significantly affects dispersion with respect to the dimensions of transferred patterns. Conventionally, by taking advantage of the tendency that isolated lines become narrower as the effective coherence factor increases, a reticle film, such as shown in FIG. 2A, is used having a plurality of patterns 32 thereon each including an isolated line pattern 34, such as shown in FIG. 2B. The isolated patterns were transferred onto a photoresist film, and the dimensions of the isolated patterns on the photoresist film are measured by using a SEM (Scanning Electron Microscope). The dispersion of the effective coherence factors was then qualitatively evaluated from the dimensional dispersion between these isolated lines in different positions of the exposure field.
In the conventional method, calculation for the dispersion of the effective coherence factors involves the following problems.
First, the accuracy of evaluating the dispersion of the effective coherence factors is relatively low. Second, the dispersion of the effective coherence factors cannot be evaluated quantitatively. Third, it takes a long time to measure the dimensions of the isolated line patterns by using the SEM.
It is therefore an object of the present invention to provide a reticle system that can accurately measure the dispersion of the effective coherence factors in the exposure field of the exposure apparatus without difficulty, and a method for calculating the dispersion of the effective coherence factors.
The present invention provides a reticle system for use in measuring an effective coherent factor in an exposure field, the reticle system including: a reticle film having thereon a plurality of scale patterns each having therein a plurality of scale marks; and a shield film disposed at a spaced relationship with respect to the reticle film and having a plurality of pinholes each disposed corresponding to one of the scale patterns, each of the scale patterns is such that a straight line passing a point light source having an effective coherent factor xe2x80x9cxxe2x80x9d and a corresponding one of the pinholes passes the reticle film at a scale mark xe2x80x9cxxe2x80x9d of the each of the scale patterns.
The reticle system for use in the measurement of an effective coherence factor, according to this invention, allows for the easy and precise measurement of the dispersion of the coherence factors in the exposure field due to aberrations in the illumination optical system, as well as the displacement of the optical axes of effective point light sources, namely illumination telecentricity.
The present invention also provides a method for measuring an effective coherent factor by using the reticle system of the present invention, the method including the steps of: transferring the scale marks onto a wafer surface by using an exposure apparatus; reading a maximum scale mark among a plurality of transferred scale marks in one of the scale patterns transferred onto the wafer surface; and determining the maximum scale mark as the effective coherent factor for the one of the scale patterns.
The method of the present invention also provides easy and precise measurement of the effective coherence factor, and calculation of the dispersion of the effective coherence factors based on the results of the effective coherence factors.
In accordance with the reticle system and the method of the present invention, the patterns transferred from the respective patterns in the reticle to the sample film represent the angle of the passed light beam with respect to the line passing through the scale pattern and the pin hole. Thus, the coherence factor for each of the scale patterns can be obtained, thereby enabling the dispersion of the coherence factors to be quantitatively evaluated.